There is a class of semiconductor products that are predominantly planar and specular (flat and shiny) and it is frequently necessary to image these devices in such way that even minor deviations from planar and specular are imaged with adequate contrast. One such class of products are semiconductor wafers which may be provided with indicia that indicate, among other things, wafer number and manufacturer. These indicia are defects in the surface of the wafer and are typically a matrix of laser etched pits. These indicia are known in the art as “soft marks”. It is necessary to image these marks to read the codes at various steps along the manufacturing process.
After the devices have been singulated (generally cut by saw into individual rectangular devices), it may be necessary to inspect the edges for small chips and cracks that may propagate over time and cause premature device failure. These inspection processes are automated and use electronic imaging cameras in combination with digital electronic computers that are programmed to perform the necessary inspections, measurements and identifications.
Dark field lighting, in general, is a technique well known to those skilled in the art and is particularly useful to inspect defects on specular objects. The definition of dark field lighting is dependant upon the properties of the illumination source, its position relative to both the object and the observer, or camera, and on the properties of the object being illuminated. In order to meet the definition of dark field lighting, it is necessary that the majority of the illumination incident on the object is reflected in a direction or directions that do not enter the optical aperture of the observer or camera. Dark field illumination can be compared against bright field illumination where the majority of light is reflected directly into the camera.
With reference to FIG. 1, dark field lighting can be achieved by placing the light source such that it is pointing at the object at an angle to the line between the camera and the object. This angle has to be greater that the angle over which the object will diffuse light. If the object has a generally diffuse reflective nature, then the angle must be larger than the half-angle over which the object will distribute incident illumination by diffuse reflection. If the object is specular, that is if the object diffuses incident illumination over a small angle or with very low efficiency or both, then the angle may be very small.
It may be desirable to make the illumination source symmetric. In this case the source may be manufactured in an annular shape and placed coaxial to the optical axis, or a plurality of sources may be arranged in an annular shape. The diameter of this annulus and its proximity to the object determine the range of angles over which the illumination is incident upon the object. Such lights are known to those skilled in the art as ring lights and are variously configured to be “high angle” or “low angle.”
In imaging certain objects, it is desirable to highlight very minor features in a surface which is otherwise substantially planar and specular. These include soft marks and the edges of singulated devices. To achieve this, it is necessary to bring the illumination source as nearly on-axis with the imaging system as possible without causing the illumination source to be directly reflected into the imaging system, i.e., narrow angle. The most effective way to achieve this, as currently know, is with the aid of baffles and providing a particular alignment between the illumination source, the object, the baffles, and the imaging system.
An implementation of this can be seen in commonly assigned U.S. Pat. No. 5,737,122 entitled Illumination System for OCR Of Indicia on A Substrate (hereinafter the '122 patent). An illustration of one embodiment of the '122 patent is shown at FIG. 2. The '122 patent describes that the axis of the camera and the axis of the lighting module are at complimentary acute angles symmetric about the normal to a specular object. The narrow-angle dark field lights are positioned close to the optical axis and are prevented from being directly imaged by the camera by baffles placed in the imaging path to prevent this. The position of the baffles restricts the field of view of the imager, but this is considered an acceptable compromise. In these designs, the narrow angle dark field lighting is composed of two opposed banks and has only one-fold symmetry. When imaging generally circular pits on wafers that make up the symbology of the lot code or serial number, this one-fold symmetry can result in a degraded image.